A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces. When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head that is positioned over a specific location of a disk by an actuator.
A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. Write heads make use of the electricity flowing through a coil, which produces a magnetic field. Electrical pulses are sent to the write head, with different patterns of positive and negative currents. The current in the coil of the write head induces a magnetic field across the gap between the head and the magnetic disk, which in turn magnetizes a small area on the recording medium.
A perpendicular magnetic recording (PMR) system records data as magnetizations oriented perpendicular to the plane of the magnetic-recording disk. The magnetic disk has a magnetically soft underlayer covered by a thin magnetically hard top layer. The perpendicular write head has a main pole with a very small cross section at the pole tip, tapered down from the cross section along the length of the yoke from which the pole tip protrudes, and a return pole having a much larger cross section along the length. A write head may also include a wrap-around shield for assisting in focusing the magnetic field emitting from the pole tip by managing the magnetic leakage from the pole tip, and a back gap. A strong, highly concentrated magnetic field emits from the writer main pole in a direction perpendicular to the magnetic disk surface, magnetizing the magnetically hard top layer of the disk. The resulting magnetic flux then travels through the soft underlayer of the magnetic disk, returning to the return pole where it is sufficiently spread out and weak that it will not erase the signal recorded by the main pole when it passes back through the magnetically hard top layer of the disk on its way back to the return pole.
However, there remains a risk associated with a write operation, that the highly concentrated magnetic field emitted from the writer main pole tip will interfere with the data integrity of adjacent tracks (referred to as “adjacent track interference”, or “ATI”) and tracks beyond adjacent (referred to as “far track interference”, or “FTI”), typically manifesting as soft errors. Because a significant amount of magnetic flux is generated in the relatively wide cross section main pole and channeled into the very small cross section pole tip, magnetic leakage occurs which can cause ATI and FTI. While there are teams of scientists and engineers designing solutions to reduce such leakage, a couple general approaches to this issue are to optimize the design of the main pole (e.g., the geometry, the flare point, etc.) and, as mentioned, to utilize a wrap-around shield (“WAS”) to “catch” and redirect the flux leakage. A WAS is typically positioned at and wrapping around the main pole tip, and is constructed of magnetic material having a lower magnetic moment than the main pole material.